The recommended method for embedding iframes is using the <iframe> tag. It makes for better syntax and also allows you to control the iframe much better.
For example, you are able to remove an iframe’s border, control scrolling options, etc.
<iframe src="https://jsfiddle.net/o7usdL0a/embed/result/" style="width: 100%; height: 300px"
scrolling="no" marginwidth="0" marginheight="0" frameborder="0">
</iframe>
The <script async> and the <iframe> methods are both the same when it comes to site performance as they are both asynchronous. If you use the tag it will load asynchronously by default, so no need to worry about blocking code.